This invention relates to a switched-mode power supply, in particular a flyback converter, with a device for limiting the output voltage.
Flyback converters possess the property of supplying constant electrical power at their output. Therefore, the output voltage of a flyback converter is governed by the resistance level of a load resistor connected to the output. In the absence of a load resistor connected to the output, the output voltage may assume very high values. From WO 94/06177 a flyback converter is known which is built in a plug-in power supply and contains a regulator for monitoring the output voltage. Two control voltages influence the performance of this flyback converter, whereof the first voltage is proportional to the primary current. By means of an auxiliary winding of a transformer two voltages are obtained of which the one is proportional to the output voltage and the other proportional to the input voltage. From these two voltages the second control voltage is derived. The regulator is provided on the secondary side of the transformer and delivers, in the presence of too high an output voltage, via an optical coupler a signal that acts upon the second control voltage. This flyback converter is a correspondingly elaborate and expensive device.
To protect against excessive voltages at the output of a switched mode power supply, a zener diode is typically used which is connected between the output terminals of the switched mode power supply. However, when the output voltage rises severely due to a malfunction of the switched mode power supply, zener diodes may heat to a temperature causing their solder joints to melt, with the consequence that the output voltage may then assume nearly any high value. Zener diodes hence are no guarantee for the output voltage to be reliably maintained below a specific permissible maximum voltage level.
It is an object of the present invention to provide a switched mode power supply comprising a simple device for limiting the output voltage.
In a switched mode power supply of the invention, a Schottky diode is connected in reverse direction between the output terminals to provide a safeguard against excessive output voltages. When the output voltage exceeds the reverse voltage of the Schottky diode, its semiconductor structure will be irreversibly destroyed within a few milliseconds, whereupon it has a very low resistance in both current directions, thus permanently short-circuiting the output of the switched mode power supply without being thermally overloaded. When the nominal output voltage of the switched mode power supply is 12 volts, for example, a Schottky diode with a breakdown voltage of between about 25 and 30 volts can be used to make sure that in the event of a defect in the switched mode power supply its output voltage does not at any time, not even temporarily, exceed 40 volts, for example. The use of a Schottky diode connected in reverse direction between the output terminals of a switched mode power supply hence ensures that the output voltage is reliably maintained below a specific maximum voltage level.
A preferred embodiment of a switched mode power supply of the invention includes a first controllable switching device arranged in series with the primary winding of a transformer and a current measuring resistor. The control state of a second controllable switching device which drives a control input of the first controllable switching device is determined by the sum of three control voltages, of which the first is proportional to the current flowing through the current measuring resistor, the second is proportional to the input voltage, and the third is proportional to the output voltage. The second control voltage can be generated by a primary voltage divider. The third control voltage can be generated by means of an auxiliary winding of the transformer.
In the switched mode power supply of the present invention the third control voltage is also conveyed to the control input of the first controllable switching device. This has the advantage that at the end of the open state of the first controllable switching device the voltage residing at its control input is of a magnitude sufficiently high to effect rapid closing of the first controllable switching device.